The present invention provides a method of forming substantially spherical ceramic beads.
Ceramic beads are useful in a number of applications including, for example, as grinding media, catalyst supports, adsorbent materials, and pen balls. There are a number of known methods of forming ceramic beads, with the xe2x80x9cspray dryxe2x80x9d and xe2x80x9coil-dropxe2x80x9d methods being most widely used. In both of these methods, an aqueous ceramic bead forming liquid is dispensed into droplets which are then hardened. In the xe2x80x9cspray dryxe2x80x9d method, all or part of the hardening takes place in a stream of air. In the xe2x80x9coil-dropxe2x80x9d method, at least a portion of the hardening occurs while the droplets are dispersed in a water-immiscible liquid, such as mineral oil. In some cases, the ceramic beads are formed using a combination of the various aspects of the xe2x80x9cspray dryxe2x80x9d and xe2x80x9coil dropxe2x80x9d methods. Representative examples of prior art methods of forming ceramic beads are disclosed in: Elliott, U.S. Pat. No. 3,457,335; Schoonover, U.S. Pat. No. 4,318,896; Hansson et al., U.S. Pat. No. 4,621,936; Green et al., U.S. Pat. No. 4,628,040; and Johns, U.S. Pat. No. 5,484,559.
The limitations and drawbacks associated with prior art methods of forming ceramic beads are well known. One of the most significant drawbacks is the difficulty in obtaining ceramic beads having a substantially mono-modal size distribution in the absence of substantial post-bead formation processing. Many of the prior art methods of forming ceramic beads produce beads having a relatively broad distribution of bead sizes. This results in requirement for additional processing, such as classification, in order to obtain beads having only the desired size. It also results in waste.
Another limitation of prior art methods of forming ceramic beads is that the beads themselves tend not to be substantially spherical. In prior art methods, it is not uncommon to obtain a substantial number of beads that have a shape similar to a teardrop or the candy sold by the Hershey Foods Corporation as HERSHEY""S KISSES. Uniform sphericity is highly desired but difficult to obtain using prior art methods. Other limitations of prior art methods include, for example, difficulties in controlling the size of the beads formed, the materials that can be used, and the production rate.
A method is needed that can be used to form spherical ceramic beads that overcome the limitations of the prior art. Such a method should be able to produce ceramic beads having a substantially mono-modal size distribution without significant post-bead formation processing. Moreover, the beads should substantially spherical. Furthermore, the method should be suited to making beads over relatively a wide range of diameters, including beads that are less than 1 mm in diameter. Furthermore, the method should be able to produce ceramic beads having a variety of compositions at a high production rate.
The present invention provides a method of forming substantially spherical ceramic beads. The method comprises conveying an aqueous ceramic slurry to a nozzle tip that is immersed in an inert water-immiscible fluid layer, wherein the nozzle tip is spaced a predetermined distance away from a rotating disk that is also immersed in the immiscible fluid layer. The rotation of the disk in the immiscible fluid creates a shear force at the nozzle tip that dislodges droplets of the aqueous ceramic slurry from the nozzle tip into the immiscible fluid layer. Once dislodged, the droplets assume a substantially spherical shape. The droplets are permitted to pass from the immiscible fluid layer into an aqueous gelling solution wherein the droplets are converted into rigid beads. In a preferred embodiment of the invention, the rigid beads are recovered from the gelling solution, washed, and then sintered to obtain a density of greater than about 98% of theoretical density and a sphericity of greater than about 0.95. Surprisingly, the droplets produced according to the method exhibit a substantially mono-modal size distribution.
The foregoing and other features of the invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed.